1. Field of the Invention
This invention relates to surface controlled subsurface safety valves for controlling fluid flow in a well.
2. Prior Art
Valves are positioned at various subsurface locations in well flow conductors to control flow through the conductor. Many subsurface valves are surface controlled. One or more control conduits extend between the subsurface valve and the wellhead surface. Fluid is pressurized or depressurized and pumped into the control conduit. Generally, the subsurface valves are normally closed. Pressurization of control fluid above a minimal value is required to open the valve. Upon depressurization of control fluid, the valve returns to its normally closed position.
Closure of present surface controlled subsurface safety valves is resisted and delayed by various control fluid forces. When such a valve is positioned at a great depth in a well, valve closure may take as long as one hour. When a disaster occurs and time is of the essence for shutting in and controlling the well, an hour delay between initiation of valve closure and complete closure is too long.
For a surface controlled subsurface safety valve having a single control conduit, as disclosed in U.S. Pat. No. 3,703,193, three control fluid forces resist valve closure. First, a hydrostatic pressure force, proportional to valve depth, is created due to the presence of control fluid within the control conduit. Second, a fluid frictional force is created due to the required displacement of a relatively large volume of control fluid from the safety valve into the small diameter control conduit during valve closure. Third, the inertia of the control fluid, which was initially at rest, and which must be displaced back into the control conduit also resists valve closure. Utilizing dual control conduits, as disclosed in U.S. Pat. No. 3,696,868, permits the first, hydrostatic pressure force to be counterbalanced and, in effect, nullified. However, valve closure is still resisted by fluid frictional forces and the inertia of the mass of control fluid at rest. Additionally, there are extra equipment costs and handling problems whenever a well installation incorporates dual control conduits for a single subsurface valve.
U.S. Pat. No. 4,005,751 discloses controlling communication of control fluid to a main valve with a pilot valve. For the disclosed main valve control fluid pressure in excess of well fluid pressure is required to both close the main valve and open the main valve. If a disaster occurred at the wellhead and the control fluid pressure dropped, the main valve disclosed in U.S. Pat. No. 4,005,751 may not close.
U.S. Pat. No. 3,292,706 discloses a subsurface main valve and an auxiliary valve. The auxiliary valve does not increase the responsiveness of the main valve to decreases in control pressure and does not permit the closure speed of the main valve to be increased.
U.S. application Ser. No. 798,180 now U.S. Pat. No. 4,119,146 filed May 18, 1977 discloses utilizing a pilot valve to control communication of control fluid to a main valve. In a first position of the pilot valve, control fluid communicates to the main valve and may be effective to open the main valve. In a second position of the pilot valve, control fluid is prevented from communicating to the main valve and the pressure responsive actuator of the main valve becomes pressure balanced. In certain of the embodiments disclosed in that application leakage in the pilot valve would permit communication between the control conduit and the tubing bore. In other embodiments disclosed in the application dual control conduits are utilized, with their inherent disadvantages of extra costs and extra handling problems. Finally, the pilot valves disclosed in the application include two seal bores and two seal means. An imperfection in any of these four elements could result in the disclosed main valve and pilot valve being rendered inoperative.